One prior art ion implanter uses two magnetic deflectors to produce a parallel scanning beam in one dimension. See, for example, U.S. Pat. No. 4,276,477. A disadvantage of this approach is that scan rates are low, typically of the order of only one Hertz. Furthermore, this prior art machine scans after acceleration of the ion beam, thereby requiring relatively large deflector fields. There is a problem in uniformly spreading the beam with such an ion beam implanter, that is in providing a spatially uniform dosage over a semiconductor wafer or other target object or workpiece.
The prior art also includes medium current ion implanters that use two dimensional scanning of an ion beam with electrostatic deflectors. However, such systems do not produce a parallel scanning beam, produce scanning after acceleration, and produce a beam whose intensity is subject to uncontrolled fluctuation.
Further, it is known in the manufacture of integrated circuits, with ion beam implantation of a semiconductor wafer, that accurate and precise ion dosage of the semiconductor are important for proper IC performance. Faulty ion implantation typically is not detected until too late to correct. It thereby renders the wafer, or at least parts of it, worthless even after costly processing.
One of the primary objectives in commercial semiconductor processing is to achieve a high throughput in terms of wafers processed per unit time. Factors which affect throughput in ion implanters include implant time, time to exchange wafers and downtime due to malfunctions. The implant time to achieve a given ion dose can be reduced by increasing the ion beam current. However, ion beam current is limited by heat which results from the energetic ions. Inefficiency in scanning arises from the fact that the ion beam is directed at the target semiconductor wafer during only a portion of the implant time. During other portions of the implant time, the ion beam may be directed at an ion beam current detector or a beam stop. Furthermore, in mechanical scan systems, scanning of the ion beam over the target wafer is usually prevented when the mechanical drive system is accelerating or decelerating in order to avoid dose variations resulting from velocity variations. In general, the ion beam may be directed at the target wafer only 30%-60% of the total implant time, thereby requiring additional time to achieve a given dose and adversely impacting throughput.
U.S. Pat. No. 4,633,138 issued Dec. 30, 1986 to Tokiguchi et al discloses an ion implanter wherein the width of a beam scan is controlled to approximate the shape of the wafer in response to a width sensor. The wafer speed is controlled to compensate for dose variations which result from different sweep widths. U.S. Pat. No. 4,260,897 issued Apr. 7, 1981 to Bakker et al discloses a technique for ion implantation wherein the beam sweep is controlled to match the shape of the target. Curved sensors on each side of the target detect the ion beam and initiate reversal of the sweep. U.S. Pat. No. 4,421,988 issued Dec. 20, 1983 to Robertson et al discloses a technique for ion beam scanning wherein the scan width is matched to the width of the target wafer by means of a predetermined sequence of scan times.
Accordingly, it is a general object of this invention to provide improved methods and apparatus for ion implantation.
Another object of the invention is to provide an ion beam implanting method and apparatus for attaining relatively precise and accurate ion dosage of semiconductor wafers, with relatively high throughput.
It is a further object of the present invention to provide methods and apparatus for ion implantation with high efficiency beam scanning.
It is a further object of the present invention to provide methods and apparatus for ion implantation wherein a beam detector is used to measure ion beam intensity without adversely affecting ion beam scanning efficiency.
It is yet another object of the present invention to provide methods and apparatus for ion implantation wherein implantation occurs during mechanical acceleration and deceleration of the workpiece so as to provide high efficiency ion beam scanning.